Method and device for polishing gemstones

ABSTRACT

A method for polishing a gemstone includes providing a polishing surface (11), mounting the gemstone in a holder (6) of a clamp (1), and choosing a reference plane (33) so that it corresponds to the polishing surface. Then the facet polishing depth is set by bringing the gemstone into a position with respect to the reference plane (33) which the gemstone will occupy after completion of polishing the facet. At the moment when the clamp occupies a same position with respect to the polishing surface as the clamp previously occupied with respect to the reference plane, polishing is stopped.

The invention concerns a method for polishing gemstones, in particulardiamond, in which a gemstone is mounted in the holder of a clamp and apolishing depth is set in order to obtain a facet, said gemstone beingpressed by means of the clamp in the appropriate position against apolishing surface in order to obtain said facet.

In polishing gemstones with high anisotropic hardness such as diamond, acrystal direction is first sought for the polishing in which the leastresistance is encountered in grinding away the material. When thisdirection is known, a first facet of the gemstone is polished. With theexisting craft techniques, the polishing operation has to be interruptedat regular intervals in order to check the polishing depth reached andthe quality of the facet obtained. This is therefore an iterativeprocess. When a first facet has been polished, the gemstone is rotatedby means of rotation axes on the holder, in order to polish a followingfacet. The angle of rotation is determined with reference to theposition of the previously polished facet and the shape which it iswished to obtain for the gemstone.

This has the disadvantage that only a limited accuracy can be achieved,since the absolute position of the gemstone with reference to thepolishing disk is not known. This can cause difficulties since theangles between the facets often may not deviate by more than 0.05degrees.

Whenever grains with a different crystal orientation come to thepolishing surface when polishing a facet of a polycrystalline gem, thehardness of the gem in the polishing direction can increase to such anextent that it is necessary to set a new polishing direction. Since theabsolute position of the gem with respect to the polishing disk is notknown, this gives rise to a laborious process, and usually an unwantedfacet is cut. It is also relatively difficult and demands great skill inorder to obtain the correct polishing depth for a particular facet.

According to a partially automated technique, in order to accuratelydetermine the polishing depths of the different facets with respect toeach other, a metal ring is placed round the gemstone, which is thenpartially polished, in some cases together with a first facet. When afollowing facet is polished, this ring makes an electrical contact withthe cutting disk when the correct polishing depth has been reached, andthe clamp is lifted, as described in UK patents 1 171 211, 1 171 212 and1 206 937. In some cases this ring is replaced by a stop, which thenmakes the electrical contact.

This method can however only be employed for polishing gemstones withso-called circular geometry, such as for example a round brilliant. Forfancy shapes, however, this is not possible, and at the moment there areno automated methods.

The invention has as its object to overcome these disadvantages byproposing a method which permits a more precise working of the gemstoneand makes it possible to polish the gemstone automatically,independently of the desired shape of said gemstone, the accuracy ofangle is maintained, irrespective of the set polishing depth andpossible corrections made.

To this end, in a first step the gemstone is set in a position withrespect to a reference plane, where said position is substantially thesame as occupied by the gemstone with respect to said polishing surfacewhen the polishing operation for a facet is finished, said referenceplane being chosen such that it corresponds to the polishing surface;next, in a second step, the gemstone is brought opposite the polishingsurface, and the polishing operation is carried out until the momentwhen the clamp occupies the same relative position with respect to thepolishing surface as the latter occupies with respect to the referenceplane in the first step.

The invention also concerns a specific device for applying this method,with a polishing surface and a clamp, where said clamp is provided witha support block, a head with a holder for removably clamping thegemstone to be polished, and a reference organ for detecting therequired polishing depth, said holder is mounted rotatably round anX-axis which is fixed with respect to the head.

This device is characterised by the fact that it comprises an adjustmentapparatus which is provided with a means to set said clamp in a positioncorresponding to that which the clamp occupies with respect to saidpolishing surface at the moment when polishing a facet of the gemstonehas been completed.

Said means can effectively comprise a reference surface located at thesame perpendicular distance and position with respect to said supportblock as the polishing surface when the clamp interacts with saidpolishing surface and when the required polishing depth of the gemstonehas been reached.

Advantageously said reference organ is fixed to said support block.

According to a particular embodiment of the invention, said adjustmentapparatus comprises a sensor which interacts with said reference organ.

According to a preferred embodiment of the invention, said support blockcomprises adjusting organs for setting the clamp in a position, before afacet is polished, such that when a previously determined polishingdepth has been reached when polishing said gemstone, the position of theholder is known with respect to the polishing surface at the moment whensaid facet has been formed.

According to a further embodiment of the invention, said adjustmentorgan is made such that it enables said head to move with respect tosaid support block.

Other characteristics and advantages of the invention will be apparentfrom the following description of a specific embodiment of the methodand device for polishing gemstones according to the invention; thisdescription is given only by way of example and in no way limits thescope of the protection claimed; the reference numbers below refer tothe attached drawings.

FIG. 1 is a schematic side view, with partial cross-section, of a firstcomponent of a specific embodiment of the device according to theinvention.

FIG. 2 is a cross-section along line II--II in FIG. 1.

FIG. 3 is a side view, with partial cross-section, of a second componentof this specific embodiment of the device according to the invention.

FIG. 4 is a side view of an important component of a clamp according tothe invention.

FIG. 5 is a schematic side view of a part of the polishing table, with aslide, an adjustment apparatus and a clamp in partial cross-section, foranother embodiment of the device according to the invention.

In the different drawings the same reference numbers refer to the sameor analogous elements.

The embodiments of the device shown in the drawings essentially consistof a clamp 1, a slide 2, a polishing disk 11 and an adjustment apparatus3.

The clamp 1 is provided with an arm 4 which at one end has a head 5 witha holder 6 and at the other end a support block 7. The latter restsfreely on two support points 8 of the slide 2 through the intermediaryof two supports 21 which project from underneath the support block 7 andare adjustable in height. The supports 21 lie on a straight line whichperpendicularly crosses the axis 22 of the arm 4. Accordingly, the clamp1 can swivel round the straight line connecting the support points 8 inorder to hold a gemstone 9 against a polishing disk 11, where saidgemstone 9 is removably mounted in an indexing tube 10 mounted in a dop10' at the other end of the clamp 1.

In order to enable different surfaces or facets to be polished on angemstone 9, the holder 6 comprising the dop 10' with the indexing tube10 enables the gemstone 9 to be rotated in different positions withrespect to the polishing disk 11. For this purpose the holder 6 hasthree axes of rotation: an X-axis 12, also named grain shaft, is locatedat a fixed point with respect to the head 5 and, in the embodimentshown, in a plane perpendicular to said connecting line between thesupport points 8; a X-axis 13 that intersects the X-axis 12perpendicularly; and a Y-axis 14 or indexing axis, round which theindexing tube 10 in the dop 10' can turn, where said Y-axis is locatedin a plane perpendicular to the second axis 13.

The head 5 of the clamp 1 is solidly fixed to the arm 4, which is formedby a relatively stiff tube solidly fixed to a plate 15. Said plate 15 ismounted in the support block 7 such that said plate together with thearm 4 can undergo a translation with respect to the support block 7. Forthis purpose the plate 15 is led over two parallel bars 16 and 17, asshown in FIG. 2.

In order to precisely adjust the position of the plate 15, one side ofsaid plate is provided with a guide tube 18 which interacts with thecorresponding bar 17 which is provided with a screw thread and isrotatable about its axis by means of a turning knob 19. In this way, byturning the knob 19 the plate 15 together with the arm 4 and the head 5is moved with reference to the support block 7 over the bars 16 and 17.The arm 4 projects out from said support block 7 through an opening 20which is sufficiently large to permit the movement of the arm 4.

In order to support the clamp 1 or raise it up when tilting, said clampis provided with a lifting pin 23 which can interact with the slide 2.For this purpose a lifting bar 24 projects along the axis 22 of the arm4, said lifting bar being fixed at its end to support block 7. Thelifting pin 23 is fixed to the other end of said lifting bar 24, suchthat said lifting pin 23 stands perpendicular to the axis 22 andprojects downwards through an opening in the arm 4, past the arm. Thelifting bar 24 is executed so that it has less stiffness than the arm 4.This ensures that when the gemstone 9 is brought into contact with thepolishing disk 11 during polishing, part of the weight of the clamp 1 iscontinually supported by the lifting pin 23, so that the gemstone 9 onlyhas to bear a small part of this weight. In this way, when polishingsmall facets on the gemstone 9, a slight pressure can be applied, makingit possible to work with great accuracy.

Further, the support block 7 is provided with a reference organ 25. Inthe embodiments of the clamp 1 shown in the drawings, according to theinvention, the reference organ 25 is formed by a projection solidlyattached to the support block 7 on the side opposite the head 5. Thisreference organ 25 interacts with the slide 2 or the adjustmentapparatus 3 and measures the position of the clamp 1.

In FIG. 1, the clamp 1 is shown together with the slide 2. This slide 2rests on a foot 26 whose height and inclination are adjustable by meansof three adjusting screws (not shown), where said foot 26 is placed on aworkbench (not shown in this drawing) with a polishing disk 11. Thisadjustability is necessary when positioning a new polishing disk 11. Onthe foot 26 are two rails (not shown in the drawing) over which theslide can move back and forth in a radial direction with respect to thepolishing disk 11.

The slide 2 is formed by a flat plate on which are the support points 8for the clamp 1, a lifting mechanism 27 and a sensor 28. Underneath, theslide 2 comprises guides which interact with the rails of the foot 26.

The lifting mechanism 27 is provided with a lever 29 which can tiltround the axis 30. In order to support the clamp 1 by means of the lever29, the latter extends along one end to underneath the lifting pin 23 inorder to interact with said lifting pin 23, so that by letting the lever29 tilt round the axis 30, the gemstone 9 can be brought towards or awayfrom the polishing disk 11.

The other end of the lever 29 interacts with a cam 31 which is rotatableround an axis 32 which stands perpendicular to the lever 29. Althoughthe lever 29 can be operated manually, the cam 31 is preferably drivenby an electric motor (not shown) and, when it rotates, raises or lowersthe lever 29.

When polishing small facets, the lever 29 continues to support thelifting pin 23 in order to distribute the weight as explained above, andin order to lift the gemstone away from the polishing disk 11 as quicklyas possible as soon as the correct polishing depth has been reached. Forpolishing larger facets, the lever is set in a position at a shorterdistance under the lifting pin 23, in order to lift the gemstone 9almost instantaneously whenever the polishing depth has been reached.

Further, the slide 2 comprises a sensor 28 which is placed so that whenthe clamp 1 rests on the slide 2, said sensor 28 is located under thereference organ 25 and can interact with it. In this way a signal isgiven whenever the reference organ 25 and the sensor 28 make contact.This happens when the clamp 1 tilts round the support points 8 at themoment when a certain polishing depth has been reached for the gemstone9.

In FIG. 3 the clamp 1 is shown resting on the adjustment apparatus 3.The polishing depth for the gemstone 9 is set by means of thisadjustment apparatus 3. This is done working with an imaginary plane orreference plane 33. Said plane 33 corresponds to the surface of thepolishing disk 11. Accordingly, the difference in height A between onthe one hand the support points 8 belonging to the adjustment apparatus3, and on the other hand the reference plane 33, is the same as betweenthe support points 8 belonging to the slide 2 and the surface of thepolishing disk 11.

Further, the adjustment apparatus 3 comprises a height adjustment organ34 that supports the holder 6 and can adjust the height of said holderwith respect to the reference plane 33. In the embodiment shown, saidheight adjustment organ 34 comprises a vertical axis 35 with a screwthread, where said axis 35 can be moved up and down by means of a turnscrew 36.

On the adjustment apparatus 3, in the neighbourhood of the gemstone 9,is an eyepiece 37 on which the reference plane 33 is indicated. In thisway a person looking into the eyepiece 37 in the direction of the arrow38 will see the gemstone 9 together with the reference plane 33, whichcorresponds to the polishing surface.

The adjustment apparatus 3 is also provided with a sensor 28 thatinteracts with the reference organ 25 and occupies the same positionwith respect to the reference plane 33 as is occupied by the analogoussensor 28 of the slide 2 with respect to the surface of the polishingdisk 11. In FIG. 3, reference B indicates the fixed distance, set inadvance on the adjustment apparatus 3, between the reference organ 25and the reference plane 33. This distance is therefore the same asbetween the reference organ 25 and the polishing disk 11, as shown inFIG. 1.

It is in this position where the distance B is reached that the X-axisof the clamp is calibrated perpendicular to the reference plane andadjusted perpendicular to the polishing disk.

To polish a rough gemstone 9, the gemstone is removably mounted in thedop 10 of the holder 6. The clamp 1 together with the gemstone 9 is thenplaced on the adjustment apparatus 3. With the help of the heightadjustment organ 34 and the eyepiece 37, the clamp 1 is tilted round thesupport points 8 until the reference plane 33 coincides with anapproximation plane for the facet of the gemstone 9 to be polished. Inthis way, the polishing depth for the gemstone 9 is determined. Thefixed axis 12 is then brought perpendicular to the reference plane 33 bymaking the arm 4 undergo a translation with respect to the support block7, so that the reference plane 33 coincides with he actual surface ofthe facet to be polished. This is done with the help of the turn knob19. The sensor 28 and the reference organ 25 are calibrated so that atthe moment when the X-axis 12 is perpendicular to the reference plane33, said sensor and reference organ make contact with each other andgive a signal which for example is processed by means of a computer.This adjusting of the clamp 1 ought nonetheless to be done iteratively.

In a following step, the adjusted clamp 1 is brought to the slide 2. Thelever 29 here has to be in the highest position so that the clamp 1rests on the supports 21 on the one hand, and on the other hand on thelever 29 through the intermediary of the lifting pin 23, such that thegemstone 9 does not touch the polishing disk 11 and the reference organ25 does not make contact with the sensor 28. Accordingly, the gemstone 9can be brought into contact with the polishing disk 11 by letting thecam 31 rotate. In order to allow this to happen gradually, even afterthe first contact with the polishing disk 11, part of the weight of theclamp 1 has to be borne by the lever 29.

During the actual polishing of the gemstone 9, the lever 29 is held at ashort distance underneath the lifting pin 23. The clamp 1 descendsfurther during polishing until the reference organ 25 makes contact withthe sensor 28. The X-axis 12 is then perpendicular to the surface of thepolishing disk 11, and the set polishing depth is then reached. A signalis instantaneously given by the sensor 28 to the electric motor whichdrives the cam 31, with the result that the gemstone 9 is lifted fromthe polishing disk 11.

In order to polish a following facet to the gemstone 9, said gemstone isrotated through the desired angle relative to the indexing axis 14. Forcircular geometries where it is only required to polish one series offacets at the same inclination to the X-axis, it is not necessary toadjust the clamp 1 on the adjustment apparatus again before polishing.For non-circular geometries, the polishing depth has to be set for eachfacet; this is done to a greater or lesser extent by suitable software.

In order to automate the polishing process, the holder 6 is driven bymeans of stepper motors, so as to obtain a controlled rotation round thefixed axis or X-axis 12, the Z-axis and the indexing axis 14. In orderfor the angle of rotation of the holder 6 round the fixed axis 12 to beknown, there is an electromagnetic switch ("Hall switch") in the head 5of the clamp 1, enabling an electrical signal to be generated wheneverthe holder assumes a particular rotation position. The rotation angle isthen calculated on the basis of the number of steps made by the motor.Said motor is located in the support block 7 and is mounted on the plate15. A drive shaft (not shown in the drawings) extends through the arm 4into the head 5, where the holder 6 being driven through theintermediary of a transmission and a slip clutch. The slip clutchensures that the holder 6 can also be rotated mutually about the X-axis12.

The Z-axis 13 is provided with a turn knob 40 by means of which theassembly formed by the indexing tube 10 and the gemstone 9 can berotated round the indexing axis 14, in order to set a suitable startingposition for polishing.

The rotation movement of the gemstone 9 round the indexing axis 14 isobtained by a stepper motor mounted above the holder 6 in the head 5 ofthe clamp 1. The movement is transmitted to the gemstone 9 in aconventional manner (not shown) by a shaft, a perpendicular geartransmission and an inclined gear transmission which transmits themovement to a worm shaft. This worm shaft engages a worm wheel that issolidly attached to the indexing tube 10 in which the gemstone 9 isattached. The construction is executed so that the worm shaft coincideswith the Z-axis 13.

In order to adjust the rotation angle of the indexing axis 14 round theZ-axis 13, on the holder 6 there is a worm shaft (not shown) with a wormwheel mounted on the dop 10'. The transmission ratios are chosen suchthat when the worm shaft is turned an angular accuracy of at least 0.1°is possible. The set angle can be read off from a scale (not shown) onthe holder 6. This rotation movement can possibly be achieved by meansof a stepper motor, which may be mounted in the head 5 or in the supportblock 7, depending on the mass distribution of the clamp 1.

The bar 17 or turn knob 19 can also be driven by an electric motor; thisis advantageous in particular when polishing stones with fancy shapes.

The power for the various electrical stepper motors in the clamp 1 issupplied via the two supports 21, at least one of which is electricallyinsulated from the support block 7. Accordingly, when the clamp rests onthe slide 2, electrical energy can be supplied via said supports 21.When however the clamp 1 is removed from the slide 2, the necessaryelectrical power is supplied by rechargeable batteries located in thesupport block 7.

Infrared sensors are build into the underneath of the support block 7,corresponding to sensors lying opposite in the slide. These sensors senddata between the slide 2 and the electronic section located in theclamp 1. These data are sent via the slide 2 to a control panel and acomputer which coordinates the operation of the various stepper motorsand sensors in the clamp 1. If there are several clamps 1 interactingwith the same or several polishing disks 11, the computer can coordinatethe operation of these various clamps 1.

In a variant of the embodiment described, the reference organ 25 of theclamp 1 and the sensor 28 on the slide 2 and one the adjustmentapparatus 3 are replaced by distance sensors (PSDs or position-sensitivedetectors), for example those marketed by the Hamatsu company. Thesedistance sensors enable the distance and the variations in distancebetween the clamp 1 and the slide 2 or setting apparatus 3 to bemeasured with great accuracy.

This embodiment has important advantages. For example, the polishingspeed of the gemstone 9 can be measured, which simplified finding afavourable polishing direction of the gemstone 9.

It is also possible to compensate for any eccentricity of the gemstone 9with respect to the indexing axis 14, which for example might resultfrom less-than-perfect mounting of the gemstone in the indexing tube 10.To achieve this, the polishing depth for a first facet of the gemstone 9is set on the adjustment apparatus 3 by means of the height adjustmentorgan 34 and the turn knob 19. The gemstone 9 is then rotated round theindexing axis 14 into a position corresponding with another facet to bepolished that is the most or least eccentric. The polishing depth forthis facet is set by means of the height adjustment organ 34. Thecorresponding distance variation, measured by the distance sensors, isthen sent to the computer together with the angular position of thegemstone. This can be done for various positions of the gemstone 9. Fromthese values, the computer calculates the corrections necessary to setthe polishing depth for the different facets. When polishing thegemstone 9 the distance variations are measured by the distance sensor,and the gemstone 9 is lifted form the polishing disk 11 whenever thecorrected polishing depth has been reached for a particular facet.

During the operation, the gemstone 9 is subject to large temperaturedifferences between one facet and another when polishing, due to thedifferent orientations of the crystal lattices in the different facets.Temperature differences of 300° C. are not uncommon. This can result ina difference of around 50 μm in the actual polishing depth of one facetwith respect to another as a result of thermal expansion. Accordingly, atemperature sensor (thermocouple or radiation thermometer) is preferablymounted in proximity to the gemstone. The values read from this are thenconverted into corrections for the polishing depth.

During polishing, a pendulum motion is imposed on the slide 2. As aresult, the slide 2 moves radially back and forth with respect to thepolishing disk, in order to obtain sufficiently polished facets and toobtain even wear on the polishing disk 11. During this oscillatingmotion the hight variations of the clamp 1 can be measured in order tocontinually monitor the surface condition of the polishing disk 11. Thependulum motion is driven by means of an electrical stepper motor.

Depending on the gemstone 9 to be polished, different embodiments of theclamp 1 can be used, for example a block clamp, a brillianting clamp ora clamp for fancy shapes. These clamps can be identified by infraredsensors (not shown) underneath the support block 7, so that duringsetting or polishing, the computer determines which type of clamp is tobe controlled, and which setting data from the computer are applicableto this type of clamp. Thanks to calibration of the distances A and Band the perpendicular setting of the X-axis at the calibrated distances,both on the adjustment apparatus and on the different slides, thevarious polishing clamps are interchangeable, and can be used andexchanged on adjustment apparatuses and slides with the samecalibration.

If desired, according to the invention the clamp can simply be usedmanually, according to the craft method.

The holder 6 of the clamp 1 shown in the drawings further comprises aU-shaped bridge designed so that the position of the indexing axis 14can vary between 0 and 90 degrees, that is, in principle, between ahorizontal and a vertical position.

In a more compact embodiment of the device according to the invention,as shown in FIG. 5, the adjustment apparatus 3 and the foot 26 of theslide 2 are incorporated in a polishing table 39. In this way, by movingthe slide 2 over the foot 26 it is possible to let the holder 6 of theclamp 1 rest on the height adjusting organ 34 of the adjustmentapparatus 3. Accordingly, no extra sensor 28 is necessary on theadjustment apparatus 3. In this compact embodiment the polishing disk 11preferably has a diameter of 200 mm instead of the 320 mm normally usedin the other embodiment. Such an embodiment can be used for polishinggirdles. These are mostly polished using smaller polishing disks whichrotate about a horizontal axis.

The form of the clamp 1 in the different embodiments is chosen such thatit is easy to use manually and differs only slightly from the known,conventional form.

The invention is of course in no way limited to the proposed embodimentsof the device according to the invention as described above and shown inthe drawings. For example, a clamp can be used in which the supportblock can be mounted on a fixed column on which it can be moved up anddown. In this specific case, the above-mentioned slide can if necessarybe left out, and any sensor 28 present is mounted directly on a fixedfoot 26. The slide can also be dispensed with in some cases when thesupport block tilts around a support. Further, it is possible for thehead 5 to be moved with respect to the arm 4, which is then solidlyconnected to the support block.

By means of the stepper motors and the various sensors, an almostunlimited number of restrictions can be imposed on the device accordingto the invention, by using a computer with appropriate software, whichcan vary according to the geometry of the clamp and the geometry of thegemstone to be polished. In this way the most diverse polishing shapescan be obtained. All movements can be carried out either manually orwith the help of a motor. The eyepiece 37 can possibly be replaced by acamera which can also function as a measurement system. The sensor 28can also be used for lining up the foot 26.

I claim:
 1. A device for polishing a facet on a gemstone (9), such as adiamond, with a polishing surface (11) and a clamp (1) provided with asupport block (7), a head (5) with a holder (6) for removably clampingthe gemstone (9) to be polished, and a reference means (25) fordetecting the required polishing depth, where said holder (6) isrotatably mounted around an X-axis (12) which is fixed with respect tothe head (5), which device being improved in that it comprises anadjustment apparatus (3) provided with positioning means (33, 34, 37)for setting said clamp (1) in a position corresponding to that whichsaid clamp occupies with respect to said polishing surface at the momentwhen polishing of a facet of the gemstone (9) is completed.
 2. A deviceaccording to claim 1, wherein said positioning means comprise areference plane (33) that is located at a distance (A) relative tosupport points (8) around which the support block is tiltable, thedistance being the same as a distance between the polishing surface (11)and the support points (8) at moment that the required polishing depthof the gemstone is reached.
 3. A device according to claim 1, whereinsaid reference means (25) is fixed to said support block (7).
 4. Adevice according to claim 1, wherein said adjustment apparatus (3)comprises a sensor (28) which interacts with said reference means (25).5. A device according to claim 1, wherein said support block (7)comprises adjusting means (17, 18, 19) for adjustably setting the clamp(1) by translating the head (5) with respect to the support block (7).6. A device according to claim 5, wherein said adjusting means areexecuted so that they permit said head (5) to be subjected to anup-and-down translation with respect to said support block (7).
 7. Adevice according to claim 1, wherein said adjusting means (17, 19) aredriven by an electric motor.
 8. A device according to claim 1, whereinsaid head (5) is solidly attached to an arm (4) which extends betweensaid head (5) and the support block (7).
 9. A device according to claim1, wherein said reference means (25) interacts with a sensor (28) todetermine the position of the clamp (1) with respect to the polishingsurface (11).
 10. A device according to claim 1, wherein said supportblock (7) is provided with at least one support (21) which rests on asupport point (8) round which the clamp (1) is tiltable.
 11. A deviceaccording to claim 10, wherein said clamp (1) is provided with a liftingpin (23) in order to support it when tilting round said support point(8), at least at the beginning of contact between the gemstone and thepolishing surface (11) and when the contact is ended.
 12. A deviceaccording to claim 11, wherein said lifting pin (23) is attached to alifting bar (24) which extends according to said arm (4) and is fixed tosaid supporting block (7).
 13. A device according to claim 12, whereinsaid lifting bar (24) has a lower degree of stiffness than said arm (4).14. A device according to claim 10, wherein it comprises a slide (2) onwhich the clamp (1) rests on the support point (8) through theintermediary of said support (21).
 15. A device according to claim 14,wherein said slide (2) is provided with a lifting mechanism (27) forsaid clamp (1).
 16. A device according to claim 15, wherein said liftingmechanism (27) comprises a lever (29) which at one end interacts with adriven cam (31) and at the other end with said lifting pin (23).
 17. Adevice according to claim 14, wherein said sensor (28) is provided onthe slide (2) in order to determine the position of the clamp (1) withrespect to the latter.
 18. A device according to claim 17, wherein saidreference means (25) contacts said sensor (28) to determine when thedesired polishing depth has been reached, said X-axis (12), fixed withrespect to the head (5), being perpendicular to the polishing surface.19. A device according to claim 14, wherein said slide (2) is movableradially with respect to the polishing disk (11).
 20. A device accordingto claim 1, wherein said holder (6) comprises two axes of rotation (13,14), which are preferably perpendicular to each other, so that agemstone (9) mounted in the holder (6) is rotatable around each of theaxes (13, 14) of the holder (6).
 21. A device according to claim 20,wherein said two axes of rotation are perpendicular to said X-axis (12),and further comprise a rotation axis (13) and an indexing axis (14)perpendicular to the rotation axis (13).
 22. A device according to claim20, wherein at least one of said axes (12, 13, 14) for rotating thegemstone (9) into a particular position is driven by an electric motor.23. A device according to claim 22, wherein said holder (6) is drivenaround said fixed axis (12) by a stepper motor, possibly with a slipclutch being provided between the holder (6) and said motor in order toenable a manual rotation of the holder (6) round said axis (12), andwhere a detector is provided on the holder (6) which receives a signalonce per revolution in order to determine the exact position of theholder (6) with respect to said axis (12).
 24. A device according toclaim 23, wherein said detector is a magnetic switch.
 25. A method forpolishing a gemstone, comprising:providing a polishing surface (11);mounting the gemstone in a holder (6) of a clamp (1); choosing areference plane (33) so that it corresponds to the polishing surface;setting a facet polishing depth, including bringing the gemstone into aposition with respect to the reference plane (33) which the gemstonewill occupy after a completion of polishing the facet onto the gemstone;and polishing the gemstone on the polishing surface until a moment whenthe clamp occupies a same position with respect to the polishing surfaceas the clamp previously occupied with respect to the reference plane.26. The method according to claim 25, including:providing an X-axisaround which the holder is rotatable relative to the clamp; wherein thestep of bringing the gemstone into a position includes bringing theX-axis perpendicular to the reference plane; and wherein the step ofpolishing includes polishing until a moment when the X-axis standsperpendicular to the polishing surface.